1. Field of the Invention
This invention relates to a safety device for a turbocharged engine, and more particularly to a safety device for a turbocharged engine which is provided with a turbocharger comprising a turbine disposed in the exhaust passage and a blower disposed in the intake passage and connected with the turbine by way of a rotary shaft and in which the throttle valve is disposed in the intake passage downstream of the blower.
2. Description of the Prior Art
In turbocharged engines, the volumetric efficiency is increased and the power output performance of the engine is improved as the supercharging pressure is increased. However, if the supercharging pressure is excessively increased, the durability of the engine is adversely affected. Therefore, protective devices must be provided to limit the supercharging pressure to a maximum value which should be determined taking into account both the power output performance and the durability of the engine. For example, there has been proposed, in Japanese Unexamined Patent Publication No. 57(1982)-122142, a technical concept in which fuel feed to the engine is interrupted to reduce the rotational speed of the engine when the supercharging pressure detected by a pressure sensor exceeds a preset maximum value, thereby limiting the supercharging pressure to the preset maximum value.
However, when the protective devices such as the means for interrupting fuel feed or alarm devices are actuated every time the detected supercharging pressure exceeds the preset maximum value without taking into account the actual operating condition of the engine, there arises a problem that the protective devices are actuated even when the supercharging pressure need not be lowered, thereby adversely affecting the power output performance of the engine and burdening the operator. This is because although it is necessary to actuate the protective devices to lower the supercharging pressure when the detected supercharging pressure continues to be higher than the preset maximum value for a certain time interval, the supercharging pressure need not be lowered when the detected supercharging pressure only temporarily exceeds the preset maximum value since temporary excessive supercharging pressure does not adversely affect the durability of the engine. The supercharging pressure and accordingly the detected supercharged pressure may well temporarily exceed the preset maximum value depending on the operating condition of the engine. For example, in a turbocharged engine in which the throttle valve is disposed in the intake passage downstream of the blower of the turbocharger and the pressure sensor is disposed between the blower and the throttle valve, the detected supercharging pressure can temporarily exceed the preset maximum value when the engine which has been stabilized at a high speed is abruptly decelerated since in such an operating condition the supercharging pressure between the blower and the throttle valve may be raised over the preset maximum value for a short time as shown in FIG. 1 because the blower continues to rotate under the inertial force for a short time after the throttle valve is closed.
On the other hand, in a turbocharged engine in which the throttle valve is disposed in the intake passage downstream of the blower and the pressure sensor is disposed downstream of the throttle valve, the detected supercharging pressure can temporarily exceed the preset maximum value when the throttle valve is opened to accelerate the engine again after abrupt deceleration which leads to the excessive supercharging pressure between the throttle valve and the blower described above since the excessive supercharging pressure flows through the throttle valve and is detected by the pressure sensor disposed downstream of the throttle valve.
Therefore, if the turbocharged engine system is arranged so that fuel feed to the engine is interrupted even when the detected supercharging pressure only temporarily exceeds the preset maximum value, there arises a significant problem regarding the power output performance of the engine that the accelerating response is lowered. For example, when the vehicle speed is abruptly decelerated by braking and shift-down during high speed travel to turn a corner and then the accelerator pedal is depressed to accelerate the vehicle again after rounding the corner, the rotational speed of the engine cannot be smoothly accelerated to the desired speed since the detected supercharging pressure detected by the sensor disposed between the throttle valve and the blower or downstream of the throttle valve has temporarily exceeded the preset maximum value upon the abrupt deceleration or immediately after the throttle valve is opened as described above and fuel feed is temporarily interrupted.
In the case that the protective device is in the form of an alarm system such as an alarm lamp or buzzer warning the operator to lower the supercharging pressure, for instance, by decelerating the vehicle, the operator is needlessly burdened and is forced into carrying out a needless operation if the alarm lamp or the alarm buzzer is actuated when the supercharging pressure need not be lowered, whereby his attention is distracted from his driving.